With specific reference to fish-farms, existing regulatory authorities recommend no specific method for slaughtering fish and as a result, some or all of the following techniques may be employed:    1) Asphyxiation—suffocating the fish by removal from water. Farmed trout are commonly “harvested” by removal from water into bins in which they suffocate. Fish farmers have started to put live fish into bins containing ice according to Bristol University's Department of Meat Animal Science. The researchers also found when fish were removed from water they can often still feel what is happening to them for almost 15 minutes at low temperatures. The researchers concluded that the practice of suffocating fish on ice could unnecessarily prolong the time to unconsciousness. (Kestin, Wotten & Gregory, 1991.)    2) Bleeding—cutting the fish gills causing death by blood loss. This method may be preceded by stunning the fish in a tank containing carbon-dioxide saturated water. Welfare concerns arise with this stunning method as the “fish try to escape violently” when put into the tank, (Kestin, Apr. 2, 1992). The fish are usually unable to move within one minute and do not lose sensibility for 4-5-minutes. Fish could therefore have their gills cut whilst still conscious if lack of movement was mistaken for unconsciousness. If gill-slitting was carried out unsatisfactorily, it is possible that fish could recover consciousness whilst bleeding. For salmon, bleeding is recommended if the fish are intended to be smoked. This ensures the blood vessels are not readily apparent in the finished product. (Shepherd & Bromage, 1988.)
Norwegian fish farmers slaughter salmon by cutting the main blood vessels located in the head. The fish are then returned to the water where they subsequently weaken and die from blood loss. (Sedgewick, 1988.)    3) Concussion—killing by a blow to the head with a small, hand-held club. This slaughter method can cause instantaneous unconsciousness in the fish if done properly. However, the potential for improper stunning and injury to the fish is considerable. (Kestin, Apr. 2, 1992.)    4) Electrocution—killing by placing fish in a large tank through which electricity is allowed to flow for a few seconds. The electrical current and its frequency has to be at just the right level to stun the fish without burning the tissue. In early trials the system used too much electricity and stunned too few fish to be commercially practical. (Anthony Browne, The Times, May 3, 2003)
The Bristol University research team concluded that currently practiced slaughter methods for farmed fish fall far short of the requirement for instantaneous unconsciousness. Concussion and electrocution methods have been suggested as having the most potential for achieving instantaneous unconsciousness in fish, (Kestin, Wotten & Gregory, 1991).
Currently, following their slaughter, fish are cleaned externally then prepared for market. In its slaughtered state, fish tissue will contain whatever toxic pollutants, parasites, bacterial and viral pathogens it is contaminated with.
It is widely recognized that intensive and stressful conditions, associated with fish farming, can predispose fish to attack from disease and parasitic infection and where diseases such as bacterial septicaemia and gill infections, and bacterial gill disease prevail.
Bacterial diseases are currently treated by the use of antibiotics mixed in with fish feed. Potential human health hazards can arise from the high incidence of farmed-fish disease and its subsequent treatment. Prolonged use of antibiotics in fish can lead to the development of drug-resistant strains of bacteria. It is feared that such drug resistance could then be transferred from fish bacteria to human bacteria in the digestive tract with potentially disastrous results. Many antibiotics that treat fish diseases, such as tetracycline and chloramphenicol, are also used in human medicine. (Shepherd & Bromage, 1998.)
Drug resistance may be unknowingly picked up by a human via the above route. If that person were to fall ill and be treated by a doctor using similar antibiotic, the drug may have been rendered less efficient or ineffective.
Another example, with regard to toxic PCB infestation, farmed salmon are fed from a global supply of fish-meal and fish-oil from small open sea fish which studies show are the source of PCB's (Polychlorinated Biphenyls) in most farmed salmon. In three independent studies scientists tested 37 fish-meal samples from six countries and found PCB contamination in nearly every sample. (Jacobs 2002, Easton 2002, and CIFA 1999.)
Humans can ingest PCB's from eating contaminated fish and there is broad multiple governmental agreement from multiple governmental agencies that consumption of PCB's are expected to cause cancer and alter brain development in humans.